In the fabrication of semiconductors and MEMS, wafers such as silicon wafers or masks are transported in transport pods or mini-environment enclosures that protect them from the pollution that is still present in the atmosphere of a cleanroom.
At present, stacks of silicon wafers having a diameter of 200 millimeters (mm) are transported in standard mechanical interface (SMIF) pods.
Stacks of silicon wafers having a diameter of 300 mm are transported in standardized front-opening universal pods (FOUPs).
Transport pods or mini-environment enclosures have also been devised that are adapted to transporting a single substrate wafer.
Semiconductor or other wafers typically remain for several weeks in a semiconductor fabrication unit between the various process steps. Throughout this time, semiconductor or other wafers need to be protected against any risk of pollution, which is why provision is made form them to be isolated from the atmosphere within cleanrooms, wafers being transported and conserved in mini-environment enclosures.
In present industrial applications, the atmosphere in a mini-environment enclosure is at atmospheric pressure. Such enclosures therefore do not require any energy source.
More recently, in order to improve the isolation and protection of wafers, proposals have been made to generate and maintain an inside atmosphere at low pressure. Unfortunately, a mini-environment enclosure needs to be disconnected during steps of being moved between different workstations. During such a disconnection period, the mini-environment enclosure must itself maintain the controlled atmosphere around the substrate it contains. That requires an energy source to be available in the mini-environment enclosure, together with a pumping system for maintaining the controlled atmosphere which is at very low pressure, i.e. at a pressure of the same order as the pressures that exist in the transfer and loading chambers leading to process chambers.
Whether or not they have means for generating and maintaining an inside atmosphere at low pressure, transport pods must necessarily be opened when inserting masks or semiconductor wafers, and also when extracting them. During open periods, polluting particles can penetrate into a transport pod, thereby degrading the quality of the atmosphere inside the transport pod. Such particles run the risk subsequently of becoming deposited on masks or semiconductor wafers contained in the transport pod.
Such pollution is harmful in the semiconductor industry since it leads to defects that spoil the quality of the microelectronic or micromechanical device being fabricated.
There is therefore a need to reduce the pollution of semiconductor wafers or masks while they remain within transport pods.